1. Field of the Invention
The present invention relates to a lateral bipolar transistor, and particularly to a lateral bipolar transistor formed on a substrate having an SOI structure in which a silicon layer is provided on an embedded insulting film layer.
2. Background Art
As disclosed in, for example, Japanese Patent Application Laid-Open No. Hei 6(1994)-151859, there has heretofore been known a structure in which a transistor is formed on an SOI (Silicon On Insulator) substrate. The SOI substrate is provided with an embedded oxide film below a silicon layer that serves as a device forming area. According to such a structure, a parasitic capacitance of the substrate can be reduced. As a result, an integrated circuit can be speeded up.
Described more specifically, the above publication discloses a lateral bipolar transistor wherein an emitter diffusion region, a base diffusion region and a collector diffusion region are formed so as to reach the embedded oxide film. The SOI substrate has been used to improve the performance of a MOS transistor. In this case, such a bipolar transistor as described above might be formed on the SOI substrate along with the MOS transistor. When the MOS transistor is formed on the SOI substrate, a source-drain region is normally formed so as to reach the embedded oxide film for the purpose of reducing junction capacitance.
According to the construction of the lateral bipolar transistor, the emitter diffusion region and the collector diffusion region can be formed only by a process for forming the MOS transistor having the source-drain region that reaches the embedded oxide film. Therefore, according to the conventional structure, the integrated circuit having the MOS transistor and the bipolar transistor provided on the same SOI substrate can be prevented from increasing in cost.
Including the above-mentioned document, the applicant is aware of the following documents as a related art of the present invention.    [Patent document 1] Japanese Patent Application Laid-Open No. Hei 6(1994)-151859    [Patent document 2] Japanese Patent Application Laid-Open No. Hei 4(1992)-207038    [Patent document 3] Japanese Patent Application Laid-Open No. Sho 63(1988)-5552    [Patent document 4] Japanese Patent Application Laid-Open No. Hei 7(1995)-153774    [Patent document 5] Japanese Patent Application Laid-Open No. 2002-368002
Meanwhile, a general bipolar transistor has a base diffusion region shaped in a well shape, and an emitter diffusion region and a collector diffusion region formed in the well. According to such a structure, the emitter diffusion region and the collector diffusion region can respectively be brought into contact with the base diffusion region even at their bottom faces in addition to their side faces. Setting such a structure makes it possible to allow a sufficient collector current to circulate and ensure sufficient gain.
In the lateral bipolar transistor, however, the emitter diffusion region and the collector diffusion region can respectively contact the base diffusion region only at their side faces. It was therefore difficult to ensure a sufficient collector current according to the conventional lateral bipolar transistors. The lateral bipolar transistor was accompanied by the problem that when a base drawing diffusion region approached the emitter diffusion region, current flowing from the emitter to the base increased and emitter efficiency was hence degraded, thereby reducing gain.